1. Field of the Invention
The present invention relates to a method of recovering from the reaction mother liquor residue the oxidation catalyst used for producing terephthalic acid by liquid-phase oxidation of an alkylbenzene such as p-xylene with a molecular oxygen-containing gas. More particularly, the invention relates to a method of recovering heavy metals such as cobalt, manganese, etc., and bromine simultaneously in high pure states in the production of terephthalic acid by the liquid-phase oxidation of an alkylbenzene using an oxidation catalyst composed of the heavy metals and bromine.
2. Description of the Prior Art
Various methods have been proposed for producing terephthalic acid industrially and among them a method of producing terephthalic acid by liquid-phase oxidation of an alkylbenzene such as p-xylene with a molecular oxygen-containing gas in the presence of heavy metals such as cobalt, manganese, etc., and bromine as catalyst using a lower aliphatic monocarboxylic acid as solvent has been widely employed as SD process.
The desired material, terephthalic acid is separated from the reaction product of the liquid-phase oxidation of alkylbenzene by means of filtration, centrifugal separation, etc., and then the reaction solvent is recovered from the reaction mother liquor by means of distillation, etc. However, since the residue thus formed after recovering the solvent contains a large amount of the oxidation catalyst, it is necessary for utilizing effectively such an expensive heavy metal as cobalt and manganese and bromine to recover the catalyst for reuse.
The aforesaid reaction mother liquor residue contains, however, a large amount of organic impurities such as the unreacted alkylbenzene, such intermediates as 4-carboxybenzaldehyde, p-toluic acid etc., and by-products of the oxidation reaction having unknown structures besides the oxidation catalyst. Furthermore, in the liquid-phase oxidation as mentioned above, a monocarboxylic acid such as acetic acid is used as the reaction solvent and also bromine is used as one component of the oxidation catalyst under severe reaction conditions as high temperature and high pressure and hence the materials of the reaction apparatus are corroded as the reaction liquid retains in the reaction system and is circulated therein, which results in the contamination of the reaction liquid with metals of the corroded materials such as iron, lead, nickel, molybdenum, tungsten, copper, chromium, zinc, cadmium, etc. Still further, terephthalic acid produced by the oxidation reaction is gradually accumulated on the inside walls of such equipment and conduit owing to the sparing solubility thereof in the reaction solvent and clogging of conduits occurs. Thus the inside of the reaction system is washed with an aqueous solution of an alkali such as sodium hydroxide, which sometimes results in the contamination of the reaction liquid with the alkali metal.
It is known that those impurities impede the oxidation reaction of the alkylbenzene to cause the reduction of the quality and the yield of terephthalic acid. Therefore, it is important in using repeatedly the oxidation catalyst to recover the oxidation catalyst from the residue of the reaction mother liquor so that the organic impurities, heavy metal impurities, alkali metal, etc., as described above do not enter the catalyst recovered.
Various methods have already been proposed for recovering such an oxidation catalyst without being accompanied by the contamination of the various impurities as described above. For example, a method of recovering the oxidation catalyst is disclosed in U.S. Pat. No. 3,341,470 in which the residue of the reaction mother liquor of the liquid-phase oxidation reaction is incinerated to oxidize the heavy metal catalyst therein, the residue thus incinerated is dissolved in a mineral acid such as sulfuric acid, etc., a reducing agent such as sodium sulfide, hydrogen sulfide, etc., is added to the solution to remove copper contained in the residue as an impurity as the precipitate of copper sulfide, the solution is, then, diluted with water followed by addition of a hydroxide to adjust the pH thereof to about 4, calcium carbonate is added to the solution to precipitate such metal impurities as iron, chromium, etc. as oxides, sodium carbonate is added further to the solution from which the aforesaid metal impurities have been removed to precipitate heavy metals such as cobalt and manganese as the precipitate of the carbonate thereof, and finally the residual solution is treated with an organic carboxylic acid to recover the heavy metals as the salt of the organic carboxylic acid.
Also, according to other method as described in U.S. Pat. No. 2,964,559 the reaction mother liquor residue containing the by-product of the oxidation reaction is extracted with water to recover the oxidation catalyst as the extract and removing insoluble solid impurities, the aqueous extract containing the oxidation catalyst is adjusted to pH 4-5 with an alkali, whereby tarry by-products are removed as floats and also iron is removed as the precipitate of basic iron acetate, and further an alkali is added to the remaining aqueous solution, whereby the heavy metals such as cobalt and manganese are recovered as the precipitate of the carbonates.
Furthermore, according to still other method as described in Japanese Pat. Application Laid Open No. 34,088/'72 the reaction mother liquor residue containing the aforesaid impurities together with the oxidation catalyst is extracted with water to remove insoluble solid impurities, the pH of the aqueous extract thus obtained is adjusted to 6.0-6.8 by the addition of an alkali to remove iron and lead as the sparingly soluble terephthalates of them, and then a carbonate such as sodium carbonate is added to the filtrate obtained, whereby cobalt and manganese are recovered as the forms of the carbonates.
Such conventional methods as described above may be effective for recovering the heavy metal catalyst such as cobalt and manganese without being contaminated with organic impurities and heavy metal impurities which are harmful to the oxidation reaction, but these methods have such troubles in industrial operation that those conventional methods require the complicated procedure of adjusting the pH of the solution and that since the precipitates of the carbonates of the heavy metals formed by such methods are generally obtained as fine particles, it takes a long period of time to recover the precipitates by filtration. Also, the components of the oxidation catalyst effectively recovered by such conventional methods are only the heavy metal components such as cobalt and menganese. Further a method wherein bromine which is other component of the oxidation catalyst can be effectively recovered together with the heavy metal components has not yet been discovered.